Field of the Invention
The present invention relates to a sheet conveying device and an image forming apparatus having a sheet skew correction function.
Description of the Related Art
Typically, in image forming apparatuses (a printer, a copying machine, a facsimile, and the like) using a electrophotographic process technology, a uniformly charged photosensitive member (for example, a photosensitive drum) is irradiated (exposed) with laser light based on image data, so that an electrostatic latent image is formed on a surface of the photosensitive member. Then, a toner is supplied to the photosensitive member on which the electrostatic latent image is formed, so that the electrostatic latent image is visualized and a toner image is formed. After the toner image directly transferred to a sheet, or indirectly transferred to a sheet through an intermediate transfer member, the toner image is heated and pressurized, so that an image is formed on the sheet.
Such image forming apparatuses include a sheet conveying section that conveys a sheet supplied from a sheet feeding section (a sheet feeding tray section, a manual feeding tray section, and the like) to an image forming section. The sheet conveying section includes a plurality of conveying roller sections such as a loop roller section and a registration roller section. It is known that, in the sheet conveying section, the sheet is conveyed in a tilted state (so-called skew), due to a slight tilt of a conveying roller axis or a difference of nip pressure (hereinafter, “conveyance nip pressure”) among the conveying roller sections.
The skew of a sheet can be corrected such that two conveying roller sections (hereinafter, referred to as “skew correction roller sections”), which are independently driven, are arranged in parallel in a sheet width direction, and conveying speeds of the respective skew correction roller sections are differentiated (for example, JP 11-20993 A, JP 11-208939 A, and JP 2000-95384 A). Such technique is called active registration system.
For example, as illustrated in FIG. 1, a first loop roller section 31A and a second loop roller section 31B function as the skew correction roller sections. To be specific, the conveying speed of the skew correction roller section (the first loop roller section 31A in FIG. 1), to which the sheet enters first, is made slower than the conveying speed of the other skew correction roller section (the second loop roller section 31B in FIG. 1), so that the sheet is rotationally moved and the skew is corrected.
Further, in JP 11-20993 A and JP 11-208939 A, the conveying roller section arranged at an upper stream side of the skew correction roller sections in a sheet conveying direction is configured to be movable in the sheet width direction, so that the rotational movement of the sheet can be smoothly performed at the time of skew correction.
In a case of a tough sheet such as a cardboard, the skew of the sheet can be corrected without any problem by providing of a speed difference between the two skew correction roller sections. However, in a case of a weak sheet such as a thin paper, the sheet is pushed toward the sheet width direction mainly at a downstream side of the skew correction roller sections in the sheet conveying direction, as illustrated in FIG. 2, and a distortion extending in the sheet conveying direction may be caused (hereinafter, referred to as “loop distortion”). Even if the conveying roller section arranged at the upper steam side of the skew correction roller sections in the sheet conveying direction is moved in the sheet width direction, like JP 11-20993 A and JP 11-208939 A, the loop distortion caused at the downstream side of the skew correction roller sections in the sheet conveying direction cannot be removed.
When the sheet in a distorted state enters the conveying roller section (a registration roller section 32 in FIG. 2) at the downstream side in the sheet conveying direction and is nipped, wrinkles occur, and the quality of the image formed product is substantially deteriorated.